In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Drilling tools have been developed that, contrary to solid drills, are composed of two parts, including a basic body or drill body and a head detachably connected with the same and thereby being replaceable. The head includes the requisite cutting edges. In such a way, the major part of the tool can be manufactured from a comparatively inexpensive material having a moderate modulus of elasticity, such as steel, while a smaller part, the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the cutting edges a good chip-removing capacity, a good machining precision and a long service life. The head forms a wear part that can be discarded after wear-out, while the basic body can be re-used several times, for example, 10 to 20 replacements. A now recognized term for these cutting edge-carrying heads is “loose tops”, which henceforth will be used in this document.
Loose top type drilling tools have a plurality of desired capabilities, one of which is that torque should be transferable in a reliable way from the rotatable, driven basic body to the loose top. Furthermore, the basic body should without problems be able to carry the rearwardly directed axial forces that the loose top is subjected to during drilling. Further, the loose top should be held centered in a reliable way in relation to the basic body. Also, the loose top should be fixedly held to the basic body not only during drilling of a hole, but also during retraction of the drilling tool out of the same. A user further desires that the loose top should be mountable and dismountable in a rapid and convenient way without the basic body necessarily having to be removed from the driving machine. In addition, the tool, and in particular the loose top manufactured from expensive materials, should be capable of low cost manufacture.
Drilling tools as well as milling tools, for example, shank-end mills, of the loose top type are widely described in the patent literature and may be divided into a number of different categories depending on the ideas on which the designs are based.
A first category (see, for example, EP 1306152, EP 1328366, EP 1555075, WO03/031104, U.S. Pat. Nos. 6,695,551, 6,783,308, 6,899,495, 7,114,892, 7,134,186 and US 2008/0193237) is based on the fact that the basic body of the tool, in the front end thereof, is formed with a slot that is positioned behind the axial support surface thereof and separated into two branches. The inherent elasticity of the material in the basic body, for example, steel can be bent, for example, by radial screws, eccentric keys, etc., and be utilized to clamp the loose top. However, for differing reasons, attempts to manufacture such loose-top tools on an industrial scale have not been successful, among other things, because the slot opens not only axially but also laterally and in such a way forms a trap in which the removed metal chips may entangle themselves and entail difficulties, for example, chip jamming.
The tools in a second category (see, for example, U.S. Pat. No. 7,360,974), which like the tools in the first-mentioned category make use of flexible or elastically bendable branches formed in the basic body for the clamping of the loose top, are based on the fact that the loose top is formed with a tightening pin that protrudes rearward from the axial contact surfaces thereof and is insertable into a cavity positioned between the branches and behind the requisite axial support surfaces. The tightening pin is partially thicker than the cavity so that the tightening pin, when being inserted into the cavity, applies a resilient clamping force to the branches with the purpose of securely pinching the pin, and thereby the loose top, in the desired position. However, these loose-top tools are associated with problems and disadvantages difficult to master, which makes industrial series manufacturing more difficult. One disadvantage is that the axial length of the loose top is considerable in relation to the diameter. Because of this, the total volume of the loose top becomes comparatively large and causes expensive consumption of material in the manufacture. Another disadvantage is that the pinch or grip between the loose top and the branches is located at the rear parts of the branches, which is behind the requisite axial support surfaces, where the ability of the branches to deflect is minimal. Therefore, the more the branches are separated during clamping, the greater becomes the risk that the front parts of the branches lose contact with the loose top. In the extreme case, it is even possible that gaps arise between the outside of the loose top and the inner support surfaces of the front parts of the branches. Another one of several further disadvantages, in addition to complicated manufacture, is that cooperating pairs of axial contact surfaces of the loose top and the corresponding axial support surfaces of the basic body are rather small and peripherally located. In such a way, the axial force transmission between the loose top and the basic body may become unreliable, and unbalances may arise and jeopardize the centering of the loose top.
Further, in the above-mentioned U.S. Pat. No. 7,360,974, a drilling tool is presented, the loose top of which per se includes convex outsides that border on the front end of the loose top. In this case, the clamping of the loose top is, however, provided for by means of a rear tightening pin, which is insertable into a cavity that is positioned behind the axial support surfaces of the basic body and delimited between two flexible branches. This means that the clamping of the loose top is effected in an area where the branches have a bending and gripping capacity that is essentially smaller than in the front portions thereof.
A loose-top tool that differs markedly from the above-mentioned tools is disclosed in EP 1013367. In this case, two flexible branches, which together with an intermediate axial support surface form the jaw in which the loose top is securely pinched, are formed as peripherally situated, finger-like material portions that protrude axially from a plane axial support surface. This loose top tool does not need to be intersected by any slot or cavity in which chips could get caught. By the lack of any operatively required tightening pin protruding rearward from the axial contact surface of the loose top, the loose top can be made fairly short in relation to the diameter, which is material-saving and cost-reducing. In addition, the axial contact surface of the loose top, as well as the axial support surface of the basic body, extend between ends that are peripherally situated, where, for example, the surfaces have a length or a diametrical dimension that is as great as the diameter of the drill.
A disadvantage of the drilling tool of EP 1013367 is, however, that the grip of the branches on the loose top risks becoming weak and unreliable. Accordingly, the branches are arranged to be turned in into pockets that are recessed in the rear part of the two convex envelope surfaces that form outsides of the bars in the loose top that are delimited by the chip flutes. Each pocket has a limited axial extension that in turn limits the maximally possible length of the branches. In addition, the two conical inner support surfaces of the branches that are clamped against the corresponding conical side contact surfaces of the loose top are situated approximately halfway between the support surface of the jaw and the free ends of the branches. This means that the inner support surfaces of the branches will securely pinch the loose top in a plane situated comparatively near the rear end thereof. Only the rear portion of the loose top is held clamped between the branches, while the front portion thereof—in which the cutting forces act strongest—lacks efficient clamping. Another negative consequence of the branches having a limited length is that the tangential contact surfaces, which have the purpose of transferring torque to cooperating surfaces in the pockets of the loose top, get a limited area. In such a way, the surface pressure may become high and cause deformation damage. Furthermore, there is a risk that the loose top will be difficult to manufacture with precision that allows the loosed top to be centered in an accurate way. Accordingly, in practice it is impossible to precision-grind the side contact surfaces positioned in the interior of the pockets.